This invention relates to stereotactic trajectory specification for use in medical procedures.
When performing various medical procedures, it is necessary to insure that a surgical tool placed within the patient is accurately positioned. For example, a surgeon may be using a needle to take a biopsy of a brain tumor. The surgeon must be certain that the needle is accurately placed within the patient's brain in order to take the biopsy. Whether a surgeon is taking a biopsy or treating a patient by trying to kill tumor cells using a surgical tool, accurate placement is necessary to minimize any damage to healthy brain tissue. The calculations necessary to insure that the surgical tool has the proper trajectory are difficult and non-linear, which may lead to inaccuracies. Determining the accuracy of the calculations is accomplished by a technique which is somewhat incomplete.
Traditional stereotactic systems, such as the popular BRW frame illustrated in FIG. 1, employ a number of arcs to achieve the number of degrees of freedom needed to access a given target in three dimensional space. The frame 10 of FIG. 1 includes various mechanical adjustable parts or numerical control elements allowing one to define different trajectories. The degrees of freedom of the BRW frame of FIG. 1 are indicated by the parameters alpha, beta, delta, and gamma.
When a surgeon wishes to perform a medical procedure upon a target within a patient, the patient is placed upon an operating table and the patient (or at least the portion of the patient, such as the head, upon which the procedure is to be performed) is secured against movement relative to a particular frame of reference. Imaging techniques are used to locate the target zone, such as a tumor, within the patient. After the surgeon locates the target using the imaging system, such as computed tomography (CT), the surgeon selects an incision point in the patient. The incision point, which would be on the patient's skull when performing a medical procedure on the brain of a patient, corresponds to the place where the surgical tool will enter the patient. The incision point is selected by the physician so as to minimize damage to the patient's healthy tissues and/or so as to avoid any critical structures within the patient.
Having determined the location of the target and the incision point within the patient, nonlinear calculations are performed so as to give the proper values for alpha, beta, delta, and gamma as to provide the proper trajectory so that a surgical tool may access the target within the patient by going through the incision point. Upon setting the numerical control elements of the BRW frame of FIG. 1 by adjustment of fasteners, such as thumb screws, the frame 10 is moved to a phantom 12 as shown in prior art FIG. 2. The phantom 12 has a carriage 14 mounted for movement along tracks or rods extending in two perpendicular directions. Mounted to the carriage 14 is a target rod 16 having a target point 18 disposed thereon. The target point or portion 18 is placed in a position such that it corresponds to the location of the actual target within the patient. This is done by moving the carriage 14 along the tracks extending in perpendicular directions and by having an arrangement to vertically adjust the target portion or tip 18. A surgical tool such as a needle 20 is placed within a corresponding holder of the frame 10. If the needle does not intersect the target portion 18, either the settings on the frame 10 or the positioning of the target portion 18 are improper. In the usual case, the needle 20 will intersect the target portion 18 and the depth of insertion of the needle 20 in order to reach target portion 18 is marked on the needle so that this depth of insertion may be duplicated when actually operating on the patient.
Having confirmed the accurate settings on the frame 10 by using the phantom 12, the surgeon or assisting medical personnel will remove the frame 10 and attached it to a base ring (not shown) secured to the patient and corresponding to the frame of reference used for the target identification. The needle 20, which was removed from frame 10 when frame 10 was attached to the base ring, is then extended through the needle holder to the proper depth and the tip of the needle would be at the target within the patient's brain or other part of his body.
More details with respect to the prior techniques for stereotactic trajectory specification are described in Robert L. Galloway et al., Stereotactic Neurosurgery, CRC Reviews in Biomedical Engineering, 18(3):207-233 (1990) and chapters 1 and 2 of Tumor Stereotaxis by Patrick J. Kelly, copyright 1991 W. B. Saunders Company.
Although the various trajectory specification techniques previously used such as the BRW frame and the improved CRW frame have allowed surgeons to locate targets within patients, they have been subject to one or more of several disadvantages. They have required non-linear calculations to determine parameters which do not have an easy intuitive relationship with the x, y, and z coordinates of the target and the incision point which may be developed from the imaging system. In other words, it is not easy to see the relationship between, for example, the beta parameter for the frame of FIG. 1 and the x coordinate of a target within the patient. Note that, if the target is a tumor or other structure having a sufficiently large volume, the surgical tool may have to be placed at several different parts of the tumor or other target zone. For each of these placements, one would require the non-linear and relatively non-intuitive calculations. Each non-linear calculation is a potential source of positional inaccuracy. A second disadvantage of the prior art technique is that the testing arrangement shown in FIG. 2 simply determines that the tip of the needle will be disposed at the target. However, the testing arrangement of FIG. 2 does not insure that the needle 20 goes through the desired incision point. Although minor various from the proper incision point may be acceptable in some situations, deviation from the proper incision point under other situations could cause the needle 20 to damage critical structures within the patient or to otherwise damage healthy tissue more than is necessary.